1. Field of the Invention
Aspects of the present invention relate to an image forming apparatus and a power transmission unit thereof, and more particularly, to an image forming apparatus having a power transmission unit that can stably transmit power to a rotating body.
2. Description of the Related Art
In general, an electrophotographic image forming apparatus scans light onto a photosensitive drum electrified to a predetermined transmitting potential to form an electro-static latent image thereon. Then, the electrophotographic image forming apparatus develops the electro-static latent image into a predetermined color toner by a developing cartridge, and transfers and fuses the toner onto a print medium to thereby print a single-color image or a multi-color image on the print medium.
As shown in FIG. 1A, a conventional electrophotographic image forming apparatus 1 includes a photosensitive drum 10 and a plurality of developing cartridges 20 that are provided along an external circumference surface of the photosensitive drum 10.
The developing cartridges 20 store black (K), yellow (Y), magenta (M), and cyan (C) toners, and each include a developing roller 23 that is exposed to the photosensitive drum 10 side. Also, the developing cartridges 20 are detachable from a main body (not shown) of the image forming apparatus 1. A driven gear 23a is provided on one end part of a rotational axis of each of the developing rollers 23.
On a side frame 3 on a side of the image forming apparatus 1 are provided a drum driving coupling 4 to drive the photosensitive drum 10, and developing device driving gears 7C, 7M, 7Y, and 7K that engage the driven gears 23a of the developing rollers 23 if the developing cartridges 20 are mounted. A type in which a color image is formed by using one photosensitive drum 10, as shown in FIG. 1A, is called a multi-pass type. In the multi-pass type, each of the developing cartridges 20 must be sequentially driven to form the color image.
Referring to FIG. 1B, only two of the four developing device driving gears 7Y and 7K of FIG. 1A have been illustrated for a convenience of description as they have the same operating configurations. The developing device driving gears 7Y and 7K engage connecting gears 6Y and 6K if power is transmitted, and disengage the connecting gears 6Y and 6K if power is blocked. The connecting gears 6Y and 6K are engaged and disengaged with/from the developing device driving gears 7Y and 7K by cams 5Y and 5K that rotate with respect to a rotating axis 5a at a proper time.
However, in the conventional image forming apparatus 1, the degree of teeth-engagement between the developing device driving gears 7C, 7M, 7Y, and 7K and the driven gears 23a may be different by a manufacturing tolerance and an assembling position tolerance of parts if each of the developing cartridges 20 is mounted. Accordingly, a strict common difference control has to be performed to stably drive the developing cartridges 20 in a single part and assembling parts, thereby raising a manufacturing cost.
Also, as shown in FIG. 1A, an interval G between the developing roller 23 and the photosensitive drum 10 differs according to the developing cartridges 20. Accordingly, the density of the toner differs according to each color. As a result, the color image formed by toners having different densities may be an inferior image (such as a distorted color).